uv vlS SpecLroscopy

P8M 309UV VIS Spectroscopy% SpecLroscopy ls Lhe sLudy of Lhe lnLeracLlon beLween maLLer and radlaLed energy Electromagnetic radiation (light) is one form ofenergy.Produced by fluctuation of electric charge andmagnetic field.Electromagnetic radiation may be defined as thepropagation of the energy in the form of simpleharmonic wave which is produced by oscillation ofelectric charge and magnetic field residing on theatom.Electromagnetic radiation (light)Electromagnetic radiation is a selfpropagating wavein space with electric and magnetic components.EN radiation has an electric and magnetic fieldcomponent which oscillate in phase perpendicular toeach other and to the direction of energypropagation.Radio waves have longest wavelength and Cammarays have shortest!Since electromagnetic radiation (light) is a wave, itcan be characterized by either a wavelength orfrequency or wave numbers.'he electromagnetic wave consists of two fluctuatingfields-one electric and the other magnetic. 'he twovectors are at right angles to one another, and both areperpendicular to the direction of travel.'he distance between two consecutive peaks ortroughs in a particular wave.Liner distance from any point on one wave to thecorresponding point on the adjacent wave.!ncreasing wavelength/1 > /2 > /3&4/63 lambda (/)5; meter (m), practically measured in micrometers (m, a millionth of a meter) or nanometers (nm, a billionth of a meter )Wavelength (/)'he number of waves which can pass through agiven point in one second.Decreasing frequencyv1 < v2 < v3&4/63 nu (v)5;: cycle/sec, may also be denoted as hertz (Hz)Frequency (v)Creater the wavelength smaller the frequency.Frequency, ; = C//Frequency u 1Wavelengthwhere C is the velocity of the light, 3.00x108ms

All types of radiations travel with the same velocity

and no medium is required for their propagation. 'heycan travel through vacuum.Wave number is defined as the total number ofwaves passing through a space of unit length (1cmor 1m).We can calculate the wave number by taking thereciprocal value of wave length of a radiation. i.e.Wave numberWave number = 1WavelengthEnergy of a particular wave of the particularradiation can also be calculated by applying therelation:EnergyE=h.; = h. (c//)Hereh = Planck's constant = 6.626x1027erg sec; = Frequency of radiationc = Velocity of EM radiation/ = Wavelength'he arrangement of all types of electromagneticradiations in order of their increasing wavelength ordecreasing frequencies is known as completeelectromagnetic spectrum.'he full electromagnetic radiation spectrum iscontinuous and each region merges slowly into thenext.For convenience of reference, definitions of thevarious spectral regions have been set by the ]ointCommittee on Nomenclature in AppliedSpectroscopy:Electromagnetic spectrumFig: Electromagnetic spectrumRegion WaveIength (nm) Far ultraviolet 10-200 Near ultraviolet 200-380 Visible 380-780 Near infrared 780-3000 Middle infrared 3000-30,000 Far infrared 30,000-300,000 Microwave 300,000-1,000,000,000!t may be defined as the analysis of chemicalsubstance by the measurement of the amount ofradiation absorbed by this substance.Absorption spectroscopy refers to a range oftechniques employing the interaction ofelectromagnetic radiation (ENR) with matter.!n absorption spectroscopy, the intensity of a beamof light measured before and after interaction with asample is compared.Absorption spectroscopy!nfrared spectroscopyNear infrared spectroscopyNicrowave spectroscopyXray spectroscopy (emission spectroscopy)Uvvisible spectroscopyNNR spectroscopyExamples of absorption spectroscopyAbsorption spectroscopy is widely used for bothqualitative (is a chemical present?) andquantitative (how much?) and structural (is itdegraded?) work in a wide range of fields.'he Uv light below 200nm is relatively uninformativeand difficult to measure. Since air also absorbradiation in this region.!n order to measure absorption of Uv radiation below200nm, air must be removed from the instrument.'his region is called vacuum Uv region.!t can excites sigma bond electrons.'he Uv light above 200nm excites pi bond electronsand the region give informative spectra and can beeasily measured.vacuum ultraviolet region3 kinds of electrons in organic molecules are involvedUv or visible spectroscopy Bonding electron (ground level) Nonbonding electron (ground level) Antibonding electron (excited level)Electrons of ground states are in outer most shell orvalence electrons.Kinds of electron involvedWhile two atoms form chemical bond, their atomicorbital combine together to form molecular orbital.Bonding orbital and antibonding orbitalBonding orbital energy level is always lower than thatof the original atomic orbitalAntibonding orbital energy higherElectrons which are involved in bond formation.'hey are present in the lower energy level in themolecule.'hey are of two types:bond electrons (electrons in the single bond,electrons in saturated compounds)nbond electrons (electrons in double andtriple bond, electrons in unsaturated compounds)Bonding electronPaired electrons in outer most shell which are notinvolved in bond formation between atoms inmolecule.'hey are present in the higher energy level in themolecule than bonding electrons.'hey are represented by n.Organic compounds contains nitrogen, oxygen,halogen, sulphur.Nonbonding electronElectrons which are excited to a higher energy levelby absorbing energy (ENR) are called antibondingelectron.'hey are present in the higher energy level.'hey are represented by:Antibonding sigma (*) andAntibonding pi (n*)Antibonding electronEnergy

*nn*nUJ/JISJacuum UJ or Far UJ (z<190 nm )Electron transitions'his is the earliest method of molecularspectroscopy.A phenomenon of interaction of molecules withultraviolet and visible lights.Absorption of ENR results in electronic transition of amolecule, and electrons are promoted from groundstate to higher electronic states.Uvv!S spectroscopy 'he radiation which is absorbed has an energywhich exactly matches the energy differencebetween the ground state and the excited state. 'hese absorptions correspond to electronictransitions.At room temperature, most of the atoms, moleculesand electrons are in the lowest energy orbital calledground state.'he electron of atom (molecule) at ground state canabsorb ENR and transit to higher energy orbitalcalled excited state.Atom or molecule can absorb the radiation only whenthe energy of proton is equal to the energydifference of the two orbitals. F * 'ransitionsAn electron in a bonding orbital is excited to thecorresponding antibonding orbital. 'he energyrequired is large.For example, methane (which has only CH bonds,and can only undergo F * transitions) showsan absorbance maximum at 12S nm.Absorption maxima due to F * transitions arenot seen in typical Uvv!S spectra.n F * 'ransitionsSaturated compounds containing atoms with lonepairs (nonbonding electrons) are capable ofn* transitions.'hese transitions usually need less energy than * transitions. 'hey can be initiated by lightwhose wavelength is in the range 1S0 2S0 nm.'he number of organic functional groups withn* peaks in the Uv region is small.n F n* and n F n* 'ransitionsNost absorption spectroscopy of organiccompounds is based on transitions of n or nelectrons to the n* excited state.'hese transitions fall in an experimentallyconvenient region of the spectrum (200 700 nm).'hese transitions need an unsaturated group inthe molecule to provide the n electrons.nergy9696nAtomic orbital Atomic orbitalNolecular orbitalsOccupied levelsUnoccupied levels66

max 102S0Extending conjugation has a larger effect on 2max; shift is again to longer wavelengths2max 170 nm

HH HHEthylene (UPA name: ethene) 2max 217 nm(conjugated diene)H

HH

HHH

H H3HH

H3

H

HH2max 263 nmconjugated trieneycopene2max 505 nm11 conjugated double bondsorange-red pigment in tomatoes2max 451 nm11 conjugated double bondsBeerLambert's Law'he intensity of emitted electromagnetic radiationthrough a sample at a given wavelength decreasesexponentially with the sample thickness andconcentration.BeerLambert's Law'he BeerLambert Law is rigorously obeyed whena single species is present at relatively lowconcentrations.'he BeerLamberts Law is not obeyed: High concentrations Solute and solvent form complexes 'hermal equilibria exist between the ground stateand the excited state Fluorescent compounds are present in solutionA= log 1/'A= absorbance'= transmittance!f sample had a transmitted light throughsample of 80. What is the absorbance of thesample?Ans: 0.036'ransmittance is the percentage of incident light thatpasses through a sample of material.'9.5:4;;.50onventional SpectrophotometerSchematic of a conventional single-beam spectrophotometeronventional SpectrophotometerOptical system of a double-beam spectrophotometeronventional SpectrophotometerOptical system of a split-beam spectrophotometerSample cellsUv SpectrophotometerQuartz (crystalline silica)visible SpectrophotometerClassLight sourcesUv Spectrophotometer1. Hydrogen Cas Lamp2. Nercury Lampvisible Spectrophotometer1. 'ungsten Lamp efine: Iectromagnetic spectrum, VacuumUV region2 $how the infIuence of poIarity of soIvent ondifferent types of transitions onIy by figure inUV-Vis sppectroscopy